The purpose of this R21 proposal is to develop and validate a tissue engineering paradigm that uses cells isolated from the early staged embryonic chick heart to efficiently generate a 3-dimensional (3D) functioning myocardium termed "Engineered Early Embryonic Cardiac Tissue or EEECT". Developing embryos (fly, fish, frog, chicken, mouse, etc..) serve as unique experimental model systems for cardiovascular cell fate mapping, for gene discovery related to morphogenesis and malformations, for defining the physiology and biomechanics of morphogenesis. The cells destined to form the heart and blood vessels arise from several sources including the lateral splanchnic mesoderm, neural crest, anterior heart field, and proepicardial organ. These cells migrate, clonally proliferate, differentiate, and induce other cells along cardiovascular lineages in spatio-temporally defined morphometric patterns that are responsive to biomechanical and metabolic stresses. Cardiovascular tissue engineering has emerged as a field which is providing novel therapeutic options for the management of a wide range of diseases including structural heart disease and heart failure. Recently several research groups have succeeded in constructing Engineered Cardiac Tissues (ECTs). However, current technical barriers to the successful clinical implantation of ECTs include the limited proliferative capacity of ECTs, the logistical challenges of integrating ECTs into the highly organized multicellular and anisotropic contractile machinery of the mature myocardium, and the uncertain in-vivo "natural-history" of transplanted tissue engineered cells and tissues. Our laboratory has focused on developing novel approaches to investigate the in vivo and in vitro biomechanical regulation of the developing myocardium. We are now applying that expertise to develop a unique Engineered Early Embryonic Cardiac Tissue (EEECT) which will provide a robust in vitro model to continue our investigation of myocardial differentiation and adaptation to biomechanical load and also provide a potential in vitro source of engineered myocardium for cardiovascular repair. Our preliminary data with 3D culture of embryonic chick cardiac cells have provided a proof of principle that embryonic cardiomyocytes proliferate and differentiate in culture, respond to mechanical load, and develop contractile properties similar to native developing myocardium. We have focused on the use of embryonic cardiac cells due to their greater proliferative capacity versus neonatal and mature cells and their intrinsic ability to differentiate and adapt. Specific Aim 1. Develop and functionally characterize 3D Engineered Early Embryonic Cardiac Tissue (EEECT). Specific Aim 2. Define the impact of mechanical stress on the architecture and contractile function of EEECT. The significance of our proposal to develop EEECT from early embryonic myocardium is the unique opportunity to investigate cardiomyocyte differentiation and adaptation in a controlled biomechanical environment. Our long term goal is to evaluate EEECT as a novel biomaterial to repair the malformed or injured myocardium.